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[BOLT] Skip _init; avoiding GOT breakage for static binaries (#117751)

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_init is used during startup of binaires. Unfortunately, its
address can be shared (at least on AArch64 glibc static binaries) with a
data
reference that lives in the GOT. The GOT rewriting is currently unable
to distinguish between data addresses and function addresses. This leads
to the data address being incorrectly rewritten, causing a crash on
startup of the binary:

  Unexpected reloc type in static binary.

To avoid this, don't consider _init for being moved, by skipping it.

~We could add further conditions to narrow the skipped case for known
crashes, but as a straw man I thought it'd be best to keep the condition
as simple as possible and see if there any objections to this.~
(Edit: this broke the test
bolt/test/runtime/X86/retpoline-synthetic.test,
because _init was skipped from the retpoline pass and it has an indirect
call in it, so I include a check for static binaries now, which avoids
the test failure,
but perhaps this could/should be narrowed further?)

For now, skip _init for static binaries on any architecture; we could
add further conditions to narrow the skipped case for known crashes, but
as a straw man I thought it'd be best to keep the condition as simple as
possible and see if there any objections to this.

Updates #100096.
This commit is contained in:
Peter Waller 2024-11-28 14:59:07 +00:00 committed by GitHub
parent c4fbb6500a
commit b5ed375f9d
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20
bolt/lib/Rewrite/RewriteInstance.cpp
View File

@ -2927,6 +2927,23 @@ void RewriteInstance::handleRelocation(const SectionRef &RelocatedSection,
LLVM_DEBUG(dbgs() << "BOLT-DEBUG: ignoring relocation from data to data\n");
}
static BinaryFunction *getInitFunctionIfStaticBinary(BinaryContext &BC) {
// Workaround for https://github.com/llvm/llvm-project/issues/100096
// ("[BOLT] GOT array pointer incorrectly rewritten"). In aarch64
// static glibc binaries, the .init section's _init function pointer can
// alias with a data pointer for the end of an array. GOT rewriting
// currently can't detect this and updates the data pointer to the
// moved _init, causing a runtime crash. Skipping _init on the other
// hand should be harmless.
if (!BC.IsStaticExecutable)
return nullptr;
const BinaryData *BD = BC.getBinaryDataByName("_init");
if (!BD || BD->getSectionName() != ".init")
return nullptr;
LLVM_DEBUG(dbgs() << "BOLT-DEBUG: skip _init in for GOT workaround.\n");
return BC.getBinaryFunctionAtAddress(BD->getAddress());
}
void RewriteInstance::selectFunctionsToProcess() {
// Extend the list of functions to process or skip from a file.
auto populateFunctionNames = [](cl::opt<std::string> &FunctionNamesFile,
@ -3047,6 +3064,9 @@ void RewriteInstance::selectFunctionsToProcess() {
return true;
};
if (BinaryFunction *Init = getInitFunctionIfStaticBinary(*BC))
Init->setIgnored();
for (auto &BFI : BC->getBinaryFunctions()) {
BinaryFunction &Function = BFI.second;

43
bolt/test/AArch64/check-init-not-moved.s Normal file
View File

@ -0,0 +1,43 @@
# Regression test for https://github.com/llvm/llvm-project/issues/100096
# static glibc binaries crash on startup because _init is moved and
# shares its address with an array end pointer. The GOT rewriting can't
# tell the two pointers apart and incorrectly updates the _array_end
# address. Test checks that _init is not moved.
# RUN: llvm-mc -filetype=obj -triple aarch64-unknown-unknown %s -o %t.o
# RUN: %clang %cflags %t.o -o %t.exe -Wl,-q -static -Wl,--section-start=.data=0x1000 -Wl,--section-start=.init=0x1004
# RUN: llvm-bolt %t.exe -o %t.bolt
# RUN: llvm-nm %t.exe | FileCheck --check-prefix=CHECK-ORIGINAL %s
# RUN: llvm-nm %t.bolt | FileCheck --check-prefix=CHECK-BOLTED %s
.section .data
.globl _array_end
_array_start:
.word 0x0
_array_end:
.section .init,"ax",@progbits
.globl _init
# Check that bolt doesn't move _init.
#
# CHECK-ORIGINAL: 0000000000001004 T _init
# CHECK-BOLTED: 0000000000001004 T _init
_init:
ret
.section .text,"ax",@progbits
.globl _start
# Check that bolt is moving some other functions.
#
# CHECK-ORIGINAL: 0000000000001008 T _start
# CHECK-BOLTED-NOT: 0000000000001008 T _start
_start:
bl _init
adrp x0, #:got:_array_end
ldr x0, [x0, #:gotpage_lo15:_array_end]
adrp x0, #:got:_init
ldr x0, [x0, #:gotpage_lo15:_init]
ret